Limiting viewing angles in nuclear imaging

ABSTRACT

Methods of nuclear imaging can include, in a pre-scan, detecting radiation emitted from a patient in a first plurality of viewing angles including at least a first viewing angle and a second viewing angle, generating nuclear data from the detected radiation, reconstructing a first nuclear event distribution from the nuclear data, selecting a region of interest, determining a first signal-to-noise ratio of the first nuclear event distribution within the region of interest, selecting a second plurality of viewing angles not including the first viewing angle, reconstructing a second nuclear event distribution from the nuclear data associated with the second plurality of viewing angles, determining a second signal-to-noise ratio of the second nuclear event distribution within the region of interest, determining that the second signal-to-noise ratio is greater than or equal to the first signal-to-noise ratio, and nuclear imaging the patient by detecting nuclear data based on a nuclear imaging process that is based on the second plurality of viewing angles.

TECHNICAL FIELD

The invention relates to nuclear imaging in medicine, and in particular,to limiting viewing angles in nuclear imaging.

BACKGROUND

In nuclear imaging, one administers a radioactive substance, usually adisease specific biomarker, to a patient and detects emitted radiationwith a detector system. Examples of nuclear imaging techniques includesplanar nuclear imaging and tomographic nuclear imaging. Planar imagingis performed with a stationary imaging detector (e.g., a flat paneldetector for planar scintigraphy) that detects primarily radiationemitted towards one direction, while tomograpic imaging is performedwith detector systems that detect radiation emitted into a plurality ofdirections. Examples of tomographic nuclear imaging include, forexample, single photon emission computed tomography (SPECT) and positronemission tomography (PET). SPECT can be performed with one or severaldetectors (e.g., gamma cameras) that can be positioned or rotated aroundthe patient, while PET is usually performed with a stationary imagingdetector covering opposite sides of the patient (e.g., a ring detector).

For a nuclear event, the detector systems can detect, for example, thelocation of the respective detector pixel, the time of detection, and/orthe energy of radiation emitted by nuclear events. The detectedinformation (also referred to as nuclear data) is used to reconstruct animage of the distribution of the administered radioactive substancewithin the patient.

An overview of SPECT and PET systems and iterative image reconstructionfor emission tomography is given in chapter 7 and chapter 21 of M.Wernick and J. Aarsvold, “Emission tomography: the fundamentals of PETand SPECT,” Elsevier Academic Press, 2004, the contents of which areherein incorporated by reference. An overview of differentreconstruction methods is given in R. C. Puetter et al., “Digital ImageReconstruction: Deblurring and Denoising,” Annu. Rev. Astro. Astrophys.,2005, 43: 139-194, the contents of which are herein incorporated byreference.

SUMMARY

The invention is based in part on the recognition that in nuclearimaging, one can adapt the acquisition time according to a selectedregion of interest (ROI). The adaptation can be based on the evaluationof the signal-to-noise ratio within the ROI, which is herein referred toas the footprint signal-to-noise ratio (FSNR).

The acquisition time is an amount of time during which nuclear data isacquired. For example, the acquisition time can be a minimum acquisitiontime, i.e., the total amount of time during which nuclear events shouldat least be recorded so that the recorded nuclear data result in animage with a desired FSNR. Another example of an acquisition time thatcan be adapted is the so called dwell time, i.e., the amount of time fordetecting radiation emitted in a specific viewing angle, wherein aviewing angle usually corresponds to an angular position of a detector,and the detector may be configured to detect radiation in a range arounda particular angle as determined, for example, by the configuration ofthe detector's collimator.

For so called “point and stare” SPECT imaging, each detector position isassociated with a dwell time. For rotating systems, the dwell time canbe adjusted by the angular rotation speed of the detector. By analyzingthe contributions of the various viewing angles to the FSNR, one canadapt the dwell times such that the recorded nuclear data result in adesired FSNR. Moreover, one can analyzing the contributions of thevarious viewing angles to the FSNR to see whether specific viewingangles can be neglected during the nuclear imaging examination.

In general, in one aspect, the invention features methods of nuclearimaging that include, in a pre-scan, detecting radiation emitted from apatient in a first plurality of viewing angles including at least afirst viewing angle and a second viewing angle, generating nuclear datafrom the detected radiation, reconstructing a first nuclear eventdistribution from the nuclear data, selecting a region of interest,determining a first signal-to-noise ratio of the first nuclear eventdistribution within the region of interest, selecting a second pluralityof viewing angles not including the first viewing angle, reconstructinga second nuclear event distribution from the nuclear data associatedwith the second plurality of viewing angles, determining a secondsignal-to-noise ratio of the second nuclear event distribution withinthe region of interest, determining that the second signal-to-noiseratio is greater than or equal to the first signal-to-noise ratio, andnuclear imaging the patient by detecting nuclear data based on a nuclearimaging process that is based on the second plurality of viewing angles.

Embodiments of the methods can include one or more of the followingfeatures and/or features of other aspects.

The methods can further include selecting the first viewing angle byadaptively determining dwell times for the first plurality of viewingangles, wherein the dwell times are adapted to yield an improved nuclearevent distribution, and selecting the viewing angle associated with theshortest adaptively-determined dwell time as the first viewing angle.Adaptively determining dwell times can include improving thesignal-to-noise ratio within the region of interest by varying the dwelltimes in an optimization process. In some embodiments, the optimizationprocess can be based on a merit function for the dwell times. The meritfunction can be a function of at least one of: the plurality of viewingangles, count rates for the plurality of viewing angles, pixel weightsassociated with at least one of pixels of a detector system, dwelltimes, and the region of interest.

The methods can further include reconstructing at least one of the firstnuclear event distribution and the second nuclear event distributionincludes using a pixon reconstruction method.

The methods can further include reconstructing a nuclear image from thenuclear data acquired for the second plurality of viewing angles.

The methods can further include comprising adaptively determining dwelltimes for the second plurality of viewing angles to yield an improvednuclear event distribution from a nuclear imaging process based on thesecond plurality of viewing angles. Determining adapted dwell times caninclude improving the second signal-to-noise ratio by varying the dwelltimes in an optimization process. The optimization process can be basedon a merit function of the dwell times. In some embodiments, the meritfunction can be a function of at least one of: the plurality of viewingangles, count rates for the plurality of viewing angles, pixel weightsassociated with at least one of pixels of a detector system, dwelltimes, and the region of interest.

In general, in a further aspect, the invention features nuclear imagingapparatuses that include a detector system configured to detectradiation during a nuclear imaging process for a plurality of viewingangles and to derive nuclear data from the detected radiation, a controland reconstruction unit configured to determine signal-to-noise ratiosfor a region of interest from the nuclear data associated with at leasttwo groups of viewing angles from the plurality of viewing angles and,based on the signal-to-noise ratios, to select a group of viewing anglesfrom the at least two groups of viewing angles for a nuclear imagingprocess.

The detector system can includes at least one SPECT detector.

In some embodiments, the control and reconstruction unit can be furtherconfigured to adaptively determine dwell times for at least one of theat least two groups of viewing angles based on the signal-to-noiseratios. Each of the dwell times can be associated with a correspondingviewing angle, and each dwell time is an amount of time for collectingnuclear data from the corresponding viewing angles.

In general, in a further aspect, the invention featurescomputer-readable mediums having encoded thereon software forcontrolling a nuclear imaging system according to at least one theforegoing methods of nuclear imaging.

Embodiments of the nuclear imaging apparatuses and computer-readablemediums can also include one or more of the features of other aspects.

In some implementations, the proposed methods and systems can lead togreater efficiency in clinical nuclear medicine.

The details of one or more embodiments of the invention are set forth inthe accompanying drawings and the description below. Other features,objects, and advantages of the invention will be apparent from thedescription and drawings, and from the claims.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic view of a planar nuclear system.

FIG. 2 is a schematic view of a PET system.

FIG. 4 is a schematic view of a SPECT system.

FIG. 3 is an example of a flow chart illustrating adaptation of aminimum acquisition time.

FIG. 5 is an example of a flow chart illustrating adaptation of dwelltimes.

FIG. 6 is a schematic view illustrating limiting the number of viewingangles for SPECT imaging.

Like reference symbols in the various drawings indicate like elements.

DETAILED DESCRIPTION

Medical imaging techniques in nuclear medicine produce two-, three-, orfour-dimensional images or maps of, for example, functional processes ina patient's body by using nuclear properties of matter. For some typesof nuclear imaging, one administers a radioactive substance to thepatient and detects emitted radiation with a detector system. Thedetector system provides the recorded nuclear data to a control andreconstruction unit. Examples of detector system include a ring detectorfor PET and one or several gamma cameras for planar nuclear imaging andSPECT, respectively. Then, using especially adapted reconstructionalgorithms, the control and reconstruction unit reconstructs an imagefrom the nuclear data.

Because too much radiation can be harmful to the patient, the amount ofthe administered radioactive substance and therefore the flux ofdetected nuclear radiation, i.e. the number of counts per unit time, islimited. As a result, in nuclear imaging one often has to reconstructthe image using only a limited number of counts acquired during alimited amount of time.

The inventors realized that one can use the available amount ofacquisition time more effectively by observing statistical features ofthe image (e.g., the footprint signal-to-noise ratio (FSNR) in a regionof interest (ROI)). By doing so, one can avoid or at least reduce theradiation-inflicted harm to the patient.

In some embodiments, one can reduce the amount of time required for asingle image and thus acquire more images during a single administrationof a radioactive substance. In some embodiments, one can increase thequality of an image reconstructed from nuclear data acquired in apredefined available amount of acquisition time.

In connection with FIGS. 1-3 various nuclear imaging systems and theiroperation are described as examples of nuclear imaging systems that canbe used for adapting a nuclear imaging process based on the FSNR of aROI.

FIG. 1 shows a planar system 100 with an imaging detector 110, a controland reconstruction unit 120, and a patient table 130. Imaging detector110 can be positioned relative to a patient 140 to observe a ROI 150. InFIG. 1, for example, imaging detector 110 is positioned at the side ofpatient 140 that is close to ROI 150. Imaging detector 110 can be, forexample, a planar γ-detector with a focusing collimator system 115.

During operation, imaging detector 110 detects γ-radiation emitted froma radioactive substance administered to patient 140 and provides theresulting nuclear data to control and reconstruction unit 120.

FIG. 2 shows a PET system 200 with a stationary ring detector system210. Ring detector system 210 provides an opening 260 for receiving apatient 240 positioned on a support table 230. Ring detector system 210is configured to simultaneously detect γ-radiation (in particular, pairsof photons) that is emitted in opposing directions from an annihilationevent occurring within patient 240. The detected nuclear data isprovided to a control and reconstruction unit 220 for analysis andreconstruction.

FIG. 3 shows an example flowchart 300 of a nuclear imaging process thatuses FSNR analysis to determine a minimum acquisition time, i.e., aminimal amount of time during which nuclear data should be acquired.First, a patient is positioned on a support table and the radioactivesubstance is administered (step 310). Then, while the patient staysstationary on the support table, radioactive decay of the administeredsubstance cause the emission of radiation that is detected with nuclearimaging detectors and used to generate nuclear data to be provided to acontrol and reconstruction unit (step 320).

One then reconstructs a rough nuclear image from the nuclear data andselects a ROI (step 330). The quality of the nuclear image should allowdistinguishing a ROI from a surrounding region that is not of interest.The nuclear data can be the first nuclear data of longer nuclear imagingprocess, or it can be nuclear data that originates from a pre-imagingprocess. Alternatively, a user can select the ROI based on an imagederived from a separate imaging process (step 340). For example, one canselect the ROI based on a previously generated CT. In the latter case,the separately detected image needs to be registered in space to thefield-of-view of the nuclear imaging process.

Then, one determines a FSNR for the selected ROI (step 350) and derivesfrom that FSNR a minimum acquisition time (step 360). The minimumacquisition time is expected to allow reconstruction of a nuclear imagewithin a predefined FSNR.

The determination of the minimum acquisition time can include comparingan associated FSNR with a desired FSNR and/or performing an optimizationprocess of the minimum acquisition time for improving the FSNR. Similarto image reconstruction, the determination of the minimum acquisitiontime can use a system matrix that describes the performed imagingprocess for e.g., planar or volumetric reconstruction the nuclear image.

The nuclear imaging process can then be continued or restarted so thatnuclear data can be acquired for at least an amount of time that is aslong as the minimum acquisition time (step 370).

Based on the acquired nuclear data, one then reconstructs final image(step 380). This reconstruction can involve the system matrix and useiteratively improved data modeling. The final image can, for example, bedisplayed on a display (step 390) or provided to a diagnostic analysistool.

While FIGS. 1 and 2 refer to planar imaging and PET imaging, SPECTimaging can also profit from determining a minimum acquisition time. Anexample of a SPECT system 400 is described in connection with FIG. 4.

However, the signal-to-noise ratio for the detection process carried outby the stationary detectors of planar imaging and PET imaging depends onthe available acquisition time. The signal-to-noise ratio for SPECTimaging additionally depends on the amount of time that is available todetect radiation in specific viewing angles (dwell time).

Referring to FIG. 4, a SPECT system 400 includes a stationary unit 402and a rotating gantry ring 405. Gantry ring 405 provides an opening 460for receiving a patient 440 positioned on a support table 430 within acylindrical chamber. Gantry ring 405 carries nuclear imaging detectors410A-D mounted to a common slip ring 408. A cover 412 of gantry ring 405separates nuclear imaging detectors 410A-D from opening 460.

In SPECT, nuclear imaging detectors usually include 1 to N detectorheads rotating about the axis of the cylindrical chamber as indicated inFIG. 4 with a double-headed arrow 480. FIG. 5 shows four such detectors.

Each of nuclear imaging detectors 410A-D can be a solid state detector,such as a gamma camera that includes a direct converter and a collimator452. Nuclear imaging detectors 410A-D and associated collimators 452define a nuclear field-of-view (FOV). However, in general, various typesof detectors and collimation schemes (e.g., active or passive, parallel,focusing, multi-focusing, or coded-aperture designs) can be used.

The stationary unit of SPECT system 400 can include, for example, asupport table drive unit (not shown) and a control and reconstructionunit 420. Control and reconstruction unit 420 can control, for example,the position of patient table 430, the angular position of gantry ring405, the speed of the rotation of gantry ring 405, the type of dataacquisition with nuclear imaging detectors 410A-D, and the type of imagereconstruction.

To acquire initial nuclear data, control and reconstruction unit 420rotates gantry ring 408 around the patient either continuously orstepwise such that radiation emitted in a preset number of angularranges is detected with nuclear imaging detectors 410A-D. Depending onwhether SPECT system 400 uses a slip ring or cables to allow power totransfer to and nuclear data to transfer from nuclear imaging detectors410A-D on rotating gantry ring 405 to the stationary unit, therevolutions of rotating gantry ring 405 are either unlimited or limited(to allow the gantry to unwind the cables).

For the image reconstruction of the nuclear data, control andreconstruction unit 420 can include a processor and memory having thereconstruction code thereon. The reconstruction techniques included inthe code can take advantage, for example, of low SNR reconstruction asdescribed, for example, in U.S. patent application Ser. No. 11/931,084,by H. Vija et al. entitled “EXTERNAL PIXON SMOOTHING FOR TOMOGRAPHICIMAGE RECONSTRUCTION TECHNICAL FIELD,” filed Oct. 31, 2007 and publishedas US 2009-0110254 A1 and U.S. patent application Ser. No. 11/931,195,by H. Vija et al. entitled “RECONSTRUCTING A TOMOGRAPHIC IMAGE,” filedOct. 31, 2007 and published as US 2009-0110255 A1. The code can furtherperform an analysis of statistical features (e.g., FSNR) of theinitially detected nuclear data in a ROI 450 and thereby control thenuclear imaging process of SPECT system 400.

An example of operating a SPECT system with adapting dwell times isdescribed in the following in connection with a flowchart 500 shown inFIG. 5. First, a patient is positioned on a support table and theradioactive substance is administered (step 510). Then, while thepatient stays stationary on the support table, nuclear events of theadministered substance cause the emission of radiation that is detectedwith nuclear imaging detectors in multiple viewing angles and providedin the form of nuclear data to a control and reconstruction unit (step520).

From the nuclear data, one then reconstructs a rough nuclear image andselects a ROI (step 530). The quality of the nuclear image should allowone to distinguish a ROI from a surrounding region that is not ofinterest. This nuclear data can be the initial nuclear data of a longernuclear imaging process, or nuclear data that originates from a separatepre-imaging process. Alternatively, a user can select the ROI based onan image derived from a separate imaging process (step 540). Forexample, one can select the ROI based on a previously generated CTimage. In the latter case, the separately detected image needs to beregistered in space to the field-of-view of the SPECT system 400.

Then, one determines a FSNR for the selected ROI (step 550) and improvesa FSNR by varying the lengths of the dwell times for the variousacquisition angles (step 560). The dwell times thus determined areexpected to allow reconstruction of a nuclear image within a predefinedFSNR. The initially detected nuclear data can also be analyzed todetermine a required minimum acquisition time instead of, or in additionto adapting the dwell time.

In general, the determination of the minimum acquisition time and thedwell times can include analyzing the nuclear data with respect to a ROIand comparing an associated FSNR with a desired FSNR, modifying dwelltimes for individual viewing angles, and/or performing an optimizationprocess of the minimum acquisition time and/or dwell times for improvingthe FSNR as described below. The determination of the adapted dwelltimes, in particular, calculating the FSNR, can also involve a systemmatrix that describes the performed imaging process for, e.g., SPECTreconstruction of a nuclear image.

The nuclear imaging process can then be continued or restarted so thatfor each viewing angle, at least for an amount of time that is as longas the determined dwell time for that viewing angle, nuclear data isacquired (step 570). For continuous rotation, the adapted dwell timescan be realized by varying the rotation speed during a single rotation,while the measurement takes several rotations. For step-wise rotation,adapted dwell time can be realized by adjusting the lengths of the timeintervals during which nuclear imaging detectors 410A-D are held in aparticular angular position. In some embodiments, also the step-wiserotation performs several rotations. Using the adapted dwell times,SPECT system 400 is controlled in a manner that provides enough nucleardata to fulfill a preset FSNR within ROI 450.

Based on the nuclear data, one then reconstructs a final image (step580). This reconstruction can involve the system matrix and useiteratively improved data modeling. The final image can then bedisplayed on a display or provided to a diagnostic analysis tool (step590).

In the following, determining a minimum acquisition time and dwell timeswill be described in connection with a mathematical model.

Determining a Minimum Acquisition Time

In general, determining a minimum acquisition time is suitable for mostplanar and tomographic nuclear imaging types, including for example,SPECT and PET imaging. The determination of the minimum acquisition timecan be based on the FSNR of a ROI, where the footprint consists ofpixels of the nuclear 2D image in planar scintigraphy or of projectedpixels of a volumetric image (voxels) in tomography (SPECT or PET)within the ROI. An evaluation of the FSNR in those pixels or voxelsallows the system to adapt the minimum acquisition time to specificcondition of the nuclear examination of a patient.

The minimum acquisition time can be determined by continuouslyaccumulating and evaluating nuclear data until a desired FSNR isreached. Alternatively, one can determine the minimum acquisition timeby using the initially determined count rate as a basis for estimatingthe minimum acquisition time required to reach a desired FSNR.

Determining Dwell Times

In SPECT imaging, the dwell time at different angles can similarly beadapted to improve the nuclear imaging process. If a SPECT scan isacquired in continuous rotation mode, the rate of rotation (speedprofile) can be varied. If SPECT is performed in a step-and-shoot scan,the dwell time at each viewing angle can be varied. However, systems inwhich multiple detectors that are fixed in position with respect to eachother to simultaneously collect data from various detection angles mayhave difficulty in adapting dwell times independently for those relateddetection angles.

In general, adaptation of dwell times (adaptive dwelling) can beachieved by analyzing the contributions of different viewing angles tothe FSNR and modifying the dwell times of those viewing angles toincrease (e.g., maximize) the FSNR for a fixed minimum acquisition time.Moreover, analyzing the FSNR allows adjusting the minimum acquisitiontime and the dwell time to requirements that are specifically adjustedto the patient and medical considerations.

In the following, an example of a mathematical description of adaptingdwelling times {t_(n)} is provided. Specifically, one adapts the dwelltimes {t_(n)} of SPECT imaging based on the FSNR in a selected ROI,where n runs over the viewing angles of the detectors. At this point, aSPECT imaging mode is considered that uses a set of viewing angles inwhich the detectors are stationary (“point-and-stare” operation). Thisapproach can be generalized to the continuous imaging mode, in which thedetectors move without stopping, and the quantity to optimize is theangular velocity with which one or more detectors are rotated about thepatient.

The use of variable dwell time suggests defining object emission interms of emissivity per unit volume per unit time j (measured, say, inMBq cm⁻³), instead of the conventional total emission duringacquisition. The expected count detected at pixel i of viewing angle nthen becomes

$m_{i}^{(n)} = {t_{n}{\sum\limits_{\alpha}^{\;}{H_{i\; \alpha}^{(n)}j_{\alpha}}}}$

where H^((n)) is the system matrix for viewing angle n. When j ismeasured in MBq cm⁻³ then H^((n)) includes the voxel volume.

In order to adapt the dwell times {t_(n)}, consider a simple estimate ofthe unnormalized emissivity given by the backward projection of theexpected counts

$\begin{matrix}{{\hat{j}}_{\alpha} = {\sum\limits_{ni}^{\;}{H_{i\; \alpha}^{(n)}m_{i}^{(n)}}}} & (1)\end{matrix}$

and sum it over a region of interest (ROI) defined by the voxel weights{w_(a)}

$\hat{J} = {{\sum\limits_{\alpha}^{\;}{w_{\alpha}{\hat{j}}_{\alpha}}} = {{\sum\limits_{{ni}\; \alpha}^{\;}{H_{i\; \alpha}^{(n)}w_{\alpha}m_{i}^{(n)}}} = {\sum\limits_{ni}^{\;}{\phi_{i}^{(n)}m_{i}^{(n)}}}}}$

where the {φ_(i) ^((n))} are the pixel weights of the ROI projected intothe viewing angle n.

The optimized dwell times {t_(n)} are determined by minimizing therelative variance of Ĵ

$\begin{matrix}{\frac{V\left( \hat{J} \right)}{{E\left( \hat{J} \right)}^{2}\;} = \frac{\sum\limits_{ni}^{\;}{\left( \phi_{i}^{(n)} \right)^{2}m_{i}^{(n)}}}{\left( {\sum\limits_{ni}^{\;}{\phi_{i}^{(n)}m_{i}^{(n)}}} \right)^{2}}} & (2)\end{matrix}$

subject to a fixed acquisition time

$\begin{matrix}{t = {\sum\limits_{n}^{\;}{t_{n}.}}} & (3)\end{matrix}$

By using the relative variance, one can avoid a dependence on thenormalization of Ĵ and can use the un-normalized form that is adoptedbelow.

For static emissions (dynamic and gated studies are discussed later),the count rates (counts per unit time) {β_(i) ^((n))} do not changesignificantly with time and can be estimated from a quick pre-scan orinitial scanning period. The expected counts {m_(i) ^((n))} for eachviewing angle n are then proportional to the dwell time t_(n) to bedetermined

m _(i) ^((n)) =t _(n)β_(i) ^((n))

Taking the natural logarithm of Eq. (2) and adding a Lagrange multiplier(which turns out to be unity) to account for the constraint on availableacquisition time, the merit function to minimize becomes

$\begin{matrix}{\Lambda = {{\ln\left( {\sum\limits_{n}^{\;}{a_{n}t_{n}}} \right)} - {2\; {\ln\left( {\sum\limits_{n}^{\;}{b_{n}t_{n}}} \right)}} + {\ln\left( {\sum\limits_{n}^{\;}t_{n}} \right)}}} & (4)\end{matrix}$

where

$a_{n} = {\sum\limits_{i}^{\;}{\left( \phi_{i}^{n} \right)^{2}\beta_{i}^{(n)}}}$$b_{n} = {\sum\limits_{i}^{\;}{\phi_{i}^{n}\beta_{i}^{(n)}}}$

Eq. (4) can be solved for the dwell times {t_(n)} by standard nonlinearminimization schemes, starting with initial values that satisfy Eq. (3),for example, equal dwell times, and taking care not to change the sum ofthe dwell times during the iterative minimization.

A variant of the above scheme replaces Eq. (1) with

${\hat{j}}_{\alpha} = {\sum\limits_{ni}^{\;}{t_{n}H_{i\; \alpha}^{(n)}m_{i}^{(n)}}}$

The optimization proceeds analogously, with the merit function in Eq.(3) being replaced by

$\begin{matrix}{\Lambda = {{\ln\left( {\sum\limits_{n}^{\;}{a_{n}t_{n}^{3}}} \right)} - {2\; {\ln\left( {\sum\limits_{n}^{\;}{b_{n}t_{n}^{2}}} \right)}} + {\ln\left( {\sum\limits_{n}^{\;}t_{n}} \right)}}} & (5)\end{matrix}$

The dwell times in the above mathematical description are “net”acquisition times and do not include the setup times required for eachviewing angle.

Limiting the Viewing Angles

Tomographic reconstruction in medical applications often is based onnoisy limited-angle tomographic data, e.g., based on viewing anglesspanning less than 180° or, in general, less than that required by theuniqueness condition given in Orlov, “Theory of three dimensionalreconstruction ii: the recovery operator,” Soviet Phys. Crystallogr.,20, 429-433, 1976.

In general, the use of a reduced number of viewing angles, together witha reduction of associated overhead setup time, can decrease the minimumacquisition time.

FIG. 6 shows a schematic drawing that illustrates a detector unit 610that during SPECT imaging is kept stationary in several detectorpositions 611-617 corresponding to seven viewing angles that are used toimage a patient 650 positioned on a support table 630. For simplicity ofillustration, the number of detector positions has been limited toseven.

In a manner similar to that discussed in connection with adaptingacquisition times, one can also statistically evaluate (e.g., via theFSNR for a ROI) image reconstruction and its dependence on the number ofavailable viewing angles employed in a SPECT imaging process. One canthen reduce the number of employed viewing angles and thereby distributethe available acquisition time among fewer viewing angles by selectingthose viewing angles that contribute more to the FSNR. This procedurecan be used to increase the quality of the nuclear image and/or reducethe amount of time required for achieving a desired FSNR in a ROI. Inshort, limiting the viewing angles can speed up and/or improve nucleardata acquisition by reducing the amount of time required for detectingthe required nuclear data.

For limiting the number of viewing angles, more sophisticated imagereconstruction methods can be employed, e.g., reconstruction methodsbased on the Pixon method, as described in the foregoing mentioned U.S.Patent Applications. Those reconstruction methods can contribute to afurther reduction of viewing angles.

For example, the minimum complexity approach of the Pixon method, whichrestricts the ill-posed inversion problem, can support the reduction ofviewing angles. The key idea thereby is that the minimum complexityapproach restricts the realm of possible solutions to those that havethe minimum complexity, and thereby obtains a better guess of themissing data.

Eliminating the shortest dwell time is done in an effort to ensure thatthe extra time thus available to the other viewing angles results in asmaller relative variance V(Ĵ)/E(Ĵ)² as discussed in the foregoingmathematical description of adaptive dwelling. If it does, the viewingangle in question can be eliminated. The process is then repeated untilthe elimination of a viewing angle turns out to be counterproductive, inwhich case that viewing angle is retained, and the elimination processis halted. Referring again to FIG. 6, one can eliminate, for example,detector position 611 assuming that its contribution to the FSNR issmall in view of the distance and angle of the detector to a ROI 650 ofa patient 640. For the same reason, one may be able to eliminatedetector position 617.

The acquisition time that is saved as a result of eliminating detectorpositions can then divided among remaining detector positions 612-616.This division need not be equal. For example, detector position 614,being the closest to ROI 650, could receive a smaller part than detectorpositions 612-613 and 616-617.

The foregoing proposed adaptation of the number of viewing angles doesnot rely on but can be used with a restoration of the missing data.

A number of embodiments of the invention have been described.Nevertheless, it will be understood that various modifications may bemade without departing from the spirit and scope of the invention. Forexample, the adaptation of dwell times can be applied to various SPECToperation modes. For example, dwell-time adaptation can be applied toSPECT systems in which either the detectors or the collimatorscontinuously rotate (e.g., SPRINT concepts), to spiral SPECT scanningmodes, and to spatio-temporally consistent SPECT concepts.

While the control and reconstruction units were described to beconfigured for determining a minimum acquisition time or dwell time, thecontrol and reconstruction units can additionally be configured toperform the final image reconstruction from the acquired nuclear data.However, in some systems, the final image reconstruction may beperformed on a specialized external computer system.

The FSNR-based analysis can be performed with various imagereconstruction methods. Examples include iterative image reconstructionmethods, such as non-negative least square or Poisson-likelihoodalgorithms, that iteratively fit image models to the nuclear data. Suchreconstruction methods can further use the Pixon method, which allowsextracting information from data with a low signal-to-noise ratio. Whendetermining the ROI based on an initial reconstruction, thereconstruction using the Pixon method can support the delineation of theROI (e.g., when the signal of the ROI is determined) as well as thedetermination of the FSNR itself.

In some embodiments, the specific method by which the minimumacquisition time is determined is preordained in the operation protocolof the nuclear imaging system. In other embodiments, the specific methodis selected on a case-by-case basis. However, the adapted amount of time(minimum acquisition time and/or dwell time) is determined adaptivelybased on acquired nuclear data. In some embodiments, one can perform anupdate of the minimum acquisition time and/or dwell time during thescanning process itself. List mode reconstruction can support real timeadaptation. The update can be based on the acquired data itself or ontest data that is repeatedly acquired during the scanning process.

In some nuclear imaging systems, nuclear data can be provided in a listmode in which the detected events are recorded with coordinates positionr, detection time t, and energy E. The nuclear data can then beprocessed in real time using, for example, the Pixon method, asdescribed, for example, in the foregoing identified U.S. PatentApplications. The scanning process can then continue during theprocessing of the first initially detected nuclear data.

For both adaptive dwelling and for adaptively limiting the viewingangles, physical boundary conditions can impose further constraints onthe merit functions, e.g., the merit functions of Eqs. (4) and (5). Asmultiple detectors cannot be moved at will, the acquisition with thosedetectors may be synchronous, in which case those detectors have equaldwell times. Even with asynchronous acquisition, the minimum acquisitiontime is the same for all detectors. Those types of additionalsystem-dependent constraints can be considered when optimizing the meritfunction.

Finally, dynamic and gated imaging require further consideration. Indynamic imaging, the assumption of constant count rates is not valid. Insuch cases, the count rates {β_(i) ^((n))} need to be reevaluatedperiodically during the acquisition, so that the adaptively determineddwell times can be adjusted accordingly. One way to reevaluate the countrates is to use the count rates measured in the previous time intervalas the basis of adaptively selecting the dwell times for the next timeinterval.

In gated studies, the gates are much shorter than the dwell times.Consequently, it is not possible to change the dwell times for eachacquisition. However, in such cases one can average the count rates ofthe gates. The specific weights used in this averaging can be designedto emphasize those gates having the higher diagnostic significance.

Accordingly, other embodiments are within the scope of the followingclaims.

1. A method of nuclear imaging, the method comprising: in a pre-scan,detecting radiation emitted from a patient in a first plurality ofviewing angles including at least a first viewing angle and a secondviewing angle; generating nuclear data from the detected radiation;reconstructing a first nuclear event distribution from the nuclear data;selecting a region of interest; determining a first signal-to-noiseratio of the first nuclear event distribution within the region ofinterest; selecting a second plurality of viewing angles not includingthe first viewing angle; reconstructing a second nuclear eventdistribution from the nuclear data associated with the second pluralityof viewing angles; determining a second signal-to-noise ratio of thesecond nuclear event distribution within the region of interest;determining that the second signal-to-noise ratio is greater than orequal to the first signal-to-noise ratio; and nuclear imaging thepatient by detecting nuclear data based on a nuclear imaging processthat is based on the second plurality of viewing angles.
 2. The methodof claim 1, further comprising selecting the first viewing angle byadaptively determining dwell times for the first plurality of viewingangles, wherein the dwell times are adapted to yield an improved nuclearevent distribution and selecting the viewing angle associated with theshortest adaptively-determined dwell time as the first viewing angle. 3.The method of claim 2, wherein adaptively determining dwell timesincludes improving the signal-to-noise ratio within the region ofinterest by varying the dwell times in an optimization process.
 4. Themethod of claim 3, wherein the optimization process is based on a meritfunction for the dwell times.
 5. The method of claim 4, wherein themerit function is a function of at least one of: the plurality ofviewing angles, count rates for the plurality of viewing angles, pixelweights associated with at least one of pixels of a detector system,dwell times, and the region of interest.
 6. The method of claim 1,wherein reconstructing at least one of the first nuclear eventdistribution and the second nuclear event distribution includes using apixon reconstruction method.
 7. The method of claim 1, furthercomprising reconstructing a nuclear image from the nuclear data acquiredfor the second plurality of viewing angles.
 8. The method of claim 1,further comprising adaptively determining dwell times for the secondplurality of viewing angles to yield an improved nuclear eventdistribution from a nuclear imaging process based on the secondplurality of viewing angles.
 9. The method of claim 8, whereindetermining adapted dwell times includes improving the secondsignal-to-noise ratio by varying the dwell times in an optimizationprocess.
 10. The method of claim 9, wherein the optimization process isbased on a merit function of the dwell times.
 11. The method of claim10, wherein the merit function is a function of at least one of: theplurality of viewing angles, count rates for the plurality of viewingangles, pixel weights associated with at least one of pixels of adetector system, dwell times, and the region of interest.
 12. A nuclearimaging apparatus comprising: a detector system configured to detectradiation during a nuclear imaging process for a plurality of viewingangles and to derive nuclear data from the detected radiation; a controland reconstruction unit configured to determine signal-to-noise ratiosfor a region of interest from the nuclear data associated with at leasttwo groups of viewing angles from the plurality of viewing angles and,based on the signal-to-noise ratios, to select a group of viewing anglesfrom the at least two groups of viewing angles for a nuclear imagingprocess.
 13. The nuclear imaging apparatus of claim 12, wherein thedetector system includes at least one SPECT detector.
 14. The nuclearimaging apparatus of claim 12, wherein the control and reconstructionunit is further configured to adaptively determine dwell times for atleast one of the at least two groups of viewing angles based on thesignal-to-noise ratios.
 15. The nuclear imaging apparatus of claim 13,wherein each of the dwell times is associated with a correspondingviewing angle, and each dwell time is an amount of time for collectingnuclear data from the corresponding viewing angles.